Android中的OpenSL ES是如何实现的？_libwilhelm

一般理解的Android Audio架构是这样的：

时间长了大家也就比较认可和熟悉了，但是了解OpenSL ES之后，发现如果使用OpenSL ES，是不使用JAVA API的，那么Android系统中是哪一层实现了OpenSL ES的呢？

只要搜索OpenSL ES就会搜索到这个库如何如何在Android上使用，好的文章一大堆，比如这个 Android音频开发（6）：使用OpenSL ES API上 下，还有其它googlesamples/android-ndk中的Audio-echo等等。都是OpenSL ES往上，而不是往下。也就是是哪一层实现的OpenSL ES。我是要用我的方法查找到，并不会贴出详细的代码调用流程。

通过Audio-echo得知，要使用OpenSL ES，那么需要链接的库名称为OpenSLES，在手上的一套Android L源码中以LOCAL_MODULE := libOpenSLES查得其所在的Android.mk是frameworks/wilhelm/src/Android.mk。对应该MODULE完整的代码为

include $(CLEAR_VARS)
LOCAL_SRC_FILES := sl_entry.c sl_iid.c assert.c
LOCAL_C_INCLUDES:=                                                  \
        $(call include-path-for, wilhelm)                           \
        frameworks/av/media/libstagefright                        \
        frameworks/av/media/libstagefright/include                \
        frameworks/native/include/media/openmax
LOCAL_MODULE := libOpenSLES
LOCAL_PRELINK_MODULE := false
LOCAL_MODULE_TAGS := optional
LOCAL_CFLAGS += -x c++ -DLI_API= -fvisibility=hidden -UNDEBUG \
                -DSL_API='__attribute__((visibility("default")))'
LOCAL_SHARED_LIBRARIES := libwilhelm liblog
include $(BUILD_SHARED_LIBRARY)
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可以看到它链接到了libwilhelm中。查看libwilhelm对应的Android.mk为

include $(CLEAR_VARS)

# do not claim support for any OpenSL ES or OpenMAX AL profiles
LOCAL_CFLAGS += -DUSE_PROFILES=0

# enable API logging; details are set separately by SL_TRACE_DEFAULT below
LOCAL_CFLAGS += -DUSE_TRACE
# or -UUSE_TRACE to disable API logging

# enable mutex deadlock detection
LOCAL_CFLAGS += -DUSE_DEBUG
# or -UUSE_DEBUG for no mutex deadlock detection

# enable assert() to do runtime checking
LOCAL_CFLAGS += -UNDEBUG
# or -DNDEBUG for no runtime checking

# select the level of log messages
LOCAL_CFLAGS += -DUSE_LOG=SLAndroidLogLevel_Info
# or -DUSE_LOG=SLAndroidLogLevel_Verbose for verbose logging

# log all API entries and exits (also requires Debug or Verbose log level)
# LOCAL_CFLAGS += -DSL_TRACE_DEFAULT=SL_TRACE_ALL
# (otherwise a warning log on error results only)

# API level
LOCAL_CFLAGS += -DPLATFORM_SDK_VERSION=$(PLATFORM_SDK_VERSION)

# Reduce size of .so and hide internal global symbols
LOCAL_CFLAGS += -fvisibility=hidden -DLI_API='__attribute__((visibility("default")))'

LOCAL_SRC_FILES:=                     \
        OpenSLES_IID.c                \
        assert.c                      \
        classes.c                     \
        data.c                        \
        devices.c                     \
        entry.c                       \
        handler_bodies.c              \
        trace.c                       \
        locks.c                       \
        sles.c                        \
        sl_iid.c                      \
        sllog.c                       \
        ThreadPool.c                  \
        android/AudioPlayer_to_android.cpp    \
        android/AudioRecorder_to_android.cpp  \
        android/MediaPlayer_to_android.cpp    \
        android/OutputMix_to_android.cpp      \
        android/VideoCodec_to_android.cpp     \
        android/BufferQueueSource.cpp         \
        android/CallbackProtector.cpp         \
        android/AacBqToPcmCbRenderer.cpp      \
        android/android_AudioSfDecoder.cpp    \
        android/android_AudioToCbRenderer.cpp \
        android/android_GenericMediaPlayer.cpp\
        android/android_GenericPlayer.cpp     \
        android/android_LocAVPlayer.cpp       \
        android/android_StreamPlayer.cpp      \
        android/android_Effect.cpp            \
        android/util/AacAdtsExtractor.cpp     \
        autogen/IID_to_MPH.c                  \
        objects/C3DGroup.c                    \
        objects/CAudioPlayer.c                \
        objects/CAudioRecorder.c              \
        objects/CEngine.c                     \
        objects/COutputMix.c                  \
        objects/CMediaPlayer.c                \
        itf/IAndroidBufferQueue.c         \
        itf/IAndroidConfiguration.c       \
        itf/IAndroidEffect.cpp            \
        itf/IAndroidEffectCapabilities.c  \
        itf/IAndroidEffectSend.c          \
        itf/IAcousticEchoCancellation.c   \
        itf/IAutomaticGainControl.c       \
        itf/IBassBoost.c                  \
        itf/IBufferQueue.c                \
        itf/IDynamicInterfaceManagement.c \
        itf/IEffectSend.c                 \
        itf/IEngine.c                     \
        itf/IEngineCapabilities.c         \
        itf/IEnvironmentalReverb.c        \
        itf/IEqualizer.c                  \
        itf/IMetadataExtraction.c         \
        itf/INoiseSuppression.c           \
        itf/IMuteSolo.c                   \
        itf/IObject.c                     \
        itf/IOutputMix.c                  \
        itf/IPlay.c                       \
        itf/IPlaybackRate.c               \
        itf/IPrefetchStatus.c             \
        itf/IPresetReverb.c               \
        itf/IRecord.c                     \
        itf/ISeek.c                       \
        itf/IStreamInformation.cpp        \
        itf/IVideoDecoderCapabilities.cpp \
        itf/IVirtualizer.c                \
        itf/IVolume.c

EXCLUDE_SRC :=                            \
        sync.c                            \
        itf/I3DCommit.c                   \
        itf/I3DDoppler.c                  \
        itf/I3DGrouping.c                 \
        itf/I3DLocation.c                 \
        itf/I3DMacroscopic.c              \
        itf/I3DSource.c                   \
        itf/IAudioDecoderCapabilities.c   \
        itf/IAudioEncoder.c               \
        itf/IAudioEncoderCapabilities.c   \
        itf/IAudioIODeviceCapabilities.c  \
        itf/IDeviceVolume.c               \
        itf/IDynamicSource.c              \
        itf/ILEDArray.c                   \
        itf/IMIDIMessage.c                \
        itf/IMIDIMuteSolo.c               \
        itf/IMIDITempo.c                  \
        itf/IMIDITime.c                   \
        itf/IMetadataTraversal.c          \
        itf/IPitch.c                      \
        itf/IRatePitch.c                  \
        itf/IThreadSync.c                 \
        itf/IVibra.c                      \
        itf/IVisualization.c

LOCAL_C_INCLUDES:=                                                  \
        $(call include-path-for, wilhelm)                           \
        frameworks/av/media/libstagefright                        \
        frameworks/av/media/libstagefright/include                \
        frameworks/av/media/libstagefright/http                     \
        frameworks/native/include/media/openmax                     \
        $(call include-path-for, audio-effects)

# __pthread_gettid
LOCAL_C_INCLUDES += bionic/libc/private

LOCAL_CFLAGS += -x c++ -Wno-multichar -Wno-invalid-offsetof

LOCAL_STATIC_LIBRARIES += \
        libopensles_helper        \
        libOpenSLESUT

LOCAL_SHARED_LIBRARIES :=         \
        liblog                    \
        libutils                  \
        libmedia                  \
        libbinder                 \
        libstagefright            \
        libstagefright_foundation \
        libstagefright_http_support \
        libcutils                 \
        libgui                    \
        libdl                     \
        libeffects



LOCAL_MODULE := libwilhelm
LOCAL_MODULE_TAGS := optional

ifeq ($(TARGET_BUILD_VARIANT),userdebug)
        LOCAL_CFLAGS += -DUSERDEBUG_BUILD=1
endif

LOCAL_PRELINK_MODULE := false
include $(BUILD_SHARED_LIBRARY)
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可见该库实现了OpenSL ES所需要的entry.c。真正的实现就是该库了。这个库又属于哪一层呢？需要进一步分析。